The man who killed 26 people, including 20 children, in an attack on Sandy Hook Elementary School in Connecticut almost a year ago acted alone and his motives may never be known, according to an investigative report.

The 48-page report released by Connecticut's State Attorney said the criminal investigation into the shooting at Newtown by 20-year-old Adam Lanza is now closed and no charges will be brought.

The report revealed Lanza - who murdered his mother before the attack and ended the rampage by turning his gun on himself - was enthralled by violent video games and obsessed with mass murders, particularly the 1999 shootings at Columbine High School in Colorado.

He exhibited an array of odd behaviours, changing his clothes frequently and letting his mother do his laundry but not allowing her into his room.

He did not like holidays or being touched.

Photos taken inside his bedroom show a gun locker and the windows taped over with black garbage bags.

Investigators said there was evidence that Lanza planned the rampage, but did not discuss his plans with others.

They noted that while Lanza's significant mental health issues affected his ability to live a normal life, it remained unclear whether they contributed to his violent actions.

He had been diagnosed seven years earlier with Asperger's syndrome, but there is no scientific evidence people with the condition are more likely than anyone else to commit violent crimes.

Report reveals Lanza's mother had been worried

The report offers a portrait of Lanza's complicated relationship with his mother.

Four days earlier, she had cooked him some of his favourite meals.

But Lanza had been refusing to talk to his mother for three months, communicating only by email, even though their bedrooms were on the same floor of the house they shared in Newtown.

Lanza's mother had been worried about him, saying she could not have a job because he required her constant attention.

But there was no sign she thought he would turn violent.

She took him to shooting ranges and police found a cheque indicating she had planned to give her son a gun for Christmas.

Newtown braces for anniversary of massacre

The investigative report comes as Newtown braces for the one year anniversary of the attack.

The family of Victoria Soto, one of the teachers who was shot and killed as she tried to protect her students, says this of the document:

"Yes we have read it. No we can't make sense of why it happened. We don't know if anyone ever will."

While the large informal memorials that arose in this town of 27,000 residents in the days after the shooting have long been removed, small commemorations are sprinkled throughout the sprawling town.

In a series of emails to Newtown parents last week, John Reed, the town's interim schools superintendent, addressed the report's release and cautioned parents to be mindful of their children's' emotional well-being.

"By supporting one another, we will work our way through these challenging circumstances," Mr Reed said.

A Connecticut law passed earlier this year proscribes some evidence from the state's investigation ever being made available to the public.

The law, passed in response to the shooting, prohibits the release of photographs, film, video and other visual images showing a homicide victim if they can "reasonably be expected to constitute an unwarranted invasion of personal privacy of the victim or the victim's surviving family members."

The massacre reignited a national debate on gun laws and triggered an effort by the US president Barack Obama to tighten gun control laws.

The efforts have since failed.

Chair of the group Stop Handgun Violence, John Rosenthal, says the details in the report are staggering.

"One 20-year-old kills 26 people in less than 11 minutes. He unleashed 130 rounds. Each child, each six and seven-year-old received three to 11 rounds," he said.

"If there were pictures of what these kids looked like after receiving that much firepower from a military style weapon, they would be indistinguishable.

"And much like the Holocaust films, if Americans saw what these assault weapons were doing, believe me, we would be holding our members of Congress accountable and holding ourselves accountable."

But Mr Rosenthal says he is doubtful there will be gun reform any time soon.

"There is no purpose whatsoever for a military style weapon and high capacity ammunition clips, other than killing a lot of people quickly without having to reload," he said.

"As a gun owner, I know if I go duck hunting, by federal law I am limited to three rounds in my duck hunting shotgun, by law, to protect the duck population.

"But Congress does not put limits on how many magazines a shooter can fire or buy without even a background check in 33 states."